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<br />The trend of the data points on figure 10 show that the runoff ratios <br />increase with increasing impervious area, indicating that urbanization in- <br />creases the volume of direct runoff. The curve shown on figure 10 was <br />fitted to the data by the method of least squares. <br /> <br />Thirty-seven percent effective impervious area (Turtle Creek Basin) <br />is about the maximum for fully developed residential basins in Dallas. <br />At 37 percent effective impervious area, average annual direct runoff is <br />about double that of an undeveloped area. Sufficient data have not been <br />collected in a highly industrialized area, where the effective impervious <br />area approaches 100 percent, to determine the increase in runoff. <br /> <br />SUMMARY AND CONCLUSIONS <br /> <br />The collection of streamflow and rainfall data in and near Dallas <br />during the period 1962-70 has afforded a definition of some of the hydro- <br />logic effects attributable to urban development. A digital model of stream- <br />flow response to rainfall and evaporation input was calibrated for water- <br />sheds with different degrees of urban development as reflected by impervious <br />area. <br /> <br />The rainfall-runoff relations were used with a 57-year record of <br />rainfall to simulate annual peak discharges at 14 sites. Frequency curves <br />were then prepared from these peak discharges, and from these, the dis- <br />charges corresponding to recurrence intervals of 1.25, 2, 5, 10, 25,50, <br />and 100 years were obtained. <br /> <br />The discharges at these recurrence intervals were related to drain- <br />age area, length-root slope ratio, and percentage of impervious area by <br />multiple-regression techniques. These regional relations provide a method <br />of estimating the flood-peak characteristics at ungaged sites. Based on <br />these analyses, changing a rural basin to a fully developed residential <br />urban basin will increase the flood peak at the 2-year recurrence interval <br />by about 1.4 times, at the 10-year recurrence interval by about 1.2 times, <br />and at the 50-year recurrence interval by about 1.2 times. <br /> <br />The data indicated that runoff in a fully developed residential area <br />of about 40 percent effective impervious cover would be about double that <br />of an undeveloped area. <br /> <br />-50- <br />